Why is FileHandle inconsistently returning 'nil' - swift

Question

I have an app which is inconsistently returning 'nil' when using FileHandle to open a file for Read. I'm on OSX (10.13.4), XCode 9.4, Swift 4.1
This OSX app uses the NSOpenPanel() to get a list of files selected by the user. My 'model' class code opens these files to build a collection of data structures The code which does this starts out like this and successfully gets a FileHandle EVERY TIME for any file and is able to read data from the file.
private func getFITHeader(filename: String) {
let file: FileHandle? = FileHandle(forReadingAtPath: filename)
if file == nil {
print("FITFile >>> File open failed for file \(filename)")
}
else {
var databuffer: Data
databuffer = (file?.readData(ofLength: 80))!
:
:
}
The files also contain a block of binary data which I process in another part of the app. While I develop the code for this I'm temporarily hard coding one of the same filenames as works above for test purposes. BUT this code (below) ALWAYS throws an exception 'Thread 1: Fatal error: Unexpectedly found nil while unwrapping an Optional value' when it gets to fileHandle?.seek() - for some reason the attempt to create a FileHandle is always returning 'nil' despite the code being functionally identical to tha above.
#IBAction func btnProcFile(_ sender: Any) {
var data: Data
let filename = "/Users/johncneal/Dropbox/JN Astronomy/Astronomy/Spectroscopy/RSpec_Analyses/Gamma_Cas/20161203/Gamma Cas_065.fit"
let fileHandle: FileHandle? = FileHandle(forReadingAtPath: filename)
fileHandle?.seek(toFileOffset: 2880) //skip past the headers
let dataLenToRead = 1391 * 1039 * 2
data = (fileHandle?.readData(ofLength: dataLenToRead))!
:
:
}
The code in the second function works fine in a Playground (not attaching too much meaning to that) and, wierdly, has also worked when temporarily added to a different project. Probably also worth mentioning the length of the file path doesn't seem to matter - it behaves the same on short paths.
So the question is - why is this behaviour of FileHandle reliably inconsistent?
print()'ing the filenames presented to FileHandle() showed they were identical in each case (see below). So I'm stumped and frustrated by this - any perspectives or workarounds would be appreciated.
/Users/johncneal/Dropbox/JN Astronomy/Astronomy/Spectroscopy/RSpec_Analyses/Gamma_Cas/20161203/Gamma Cas_065.fit
/Users/johncneal/Dropbox/JN Astronomy/Astronomy/Spectroscopy/RSpec_Analyses/Gamma_Cas/20161203/Gamma Cas_065.fit

Answer 1

Found the answer - Sandboxing !!
Darren - coincidentally I did look at the URL based route and discovering it 'throws' put some proper error reporting in the catches. Low and behold they reported I didn't have permissions on the file (which initially surprised me since I'm obviously admin on my Mac's and all the files ar local and under my username.
I bit more research turned up. this article - https://forums.developer.apple.com/thread/96062 which quickly revealed its a sandboxing problem :-) Looks like recent versions of XCode have it turned on in 'Entitlements'. The post also points out that the NSOpenPanel FileOpen dialog returns 'Security scoped urls'. At first I thought this explained why the code in the first function worked but I'm not totally convinced because I was only feeding the url.path property to FileHandle.
However, turning off Sandbox in Entitlements makes everything work just fine. Yes, I know thats not the right thing to do longer term (or if I want this to go to the App Store) so I'll be checking out the right way to do this. At least I can get on now - thanks for the input.

Answer 2

The FileHandle initializers are not well named.
You should use FileHandle(forReadingFrom:URL) instead of FileHandle(forReadingAtPath:String). The former is newer API that throws an error instead of returning nil. You can use the thrown error to see why it is failing, and your variables are guaranteed to be non-nil.
For example:
#IBAction func btnProcFile(_ sender: Any) {
do {
let fileUrl = URL(fileURLWithPath:"/Users/johncneal/Dropbox/JN Astronomy/Astronomy/Spectroscopy/RSpec_Analyses/Gamma_Cas/20161203/Gamma Cas_065.fit")
let fileHandle = try FileHandle(forReadingFrom: fileUrl)
fileHandle.seek(toFileOffset: 2880) //skip past the headers
let dataLenToRead = 1391 * 1039 * 2
let data: Data = fileHandle.readData(ofLength: dataLenToRead)
// etc...
} catch let error as NSError {
print("FITFile >>> File open failed: \(error)")
NSApp.presentError(error)
}
}